This invention relates to an atmospheric vaporizer suitable for vaporizing cryogenic liquids on a substantially continuous basis at greatly improved operating efficiency.
Atmospheric gases such as oxygen, nitrogen and argon find wide use in a variety of applications. These gases are typically produced by means of air separation plants. Users of large quantities of such gases may have air separation units at the site of gas usage, while users of small quantities of such gases generally find it convenient to purchase their requirements in cylinders. Users of intermediate or moderate amounts of such gases generally do not have sufficient usage to justify an on-site air separation plant, but generally their requirements are large enough to make purchasing of gas in cylinders uneconomical. Typically, moderate users of gases will find it convenient to maintain on-site cryogenic liquid storage tanks and vaporize the liquid as requirements dictate. The gas is then piped to the use locations. The usage requirements may be intermittent or continuous.
An atmospheric vaporizer is a device which vaporizes cryogenic liquids by employing heat absorbed from the ambient air. Atmospheric vaporizers have been employed by users of intermediate quantities of gases as means of vaporizing the stored cryogenic liquid when the user's gase requirements are intermittent, but generally not when the user's gas requirements are continuous. The reason why atmospheric vaporizers are not generally used for continuous service is because ice and frost build up on the outside surfaces of the atmospheric vaporizer, rendering the unit inefficient after a sustained period of use.
Typically, an atmospheric vaporizer is comprised of one or more passes vertically positioned and piped together. The passes are comprised of a center tube through which the liquid passes, and the tube generally has one or more fins attached to it to increase the heat transfer area. The passes are spaced about 9.25 inches, centerline to centerline, from each other. The cryogenic liquid enters at the bottom of one pass, passes up through it and then through a connection to the top of another pass through which it descends. This flow pattern is repeated through other passes as conditions such as usage and ambient temperature dictate. As the cryogenic liquid passes through the atmospheric vaporizer, the liquid is vaporized and the gas then further heated by heat transferred from the ambient air to the fluid through the vaporizer. The gas exits the atmospheric vaporizer at the required flow rate and exit temperature.
As the fluid passes through the vaporizer and as heat is exchanged from the ambient air, the moisture in the air condenses and freezes on the surfaces of the vaporizer. This frost and ice continues to build up during operation of the vaporizer resulting in decreasing efficiency until a steady state condition is attained. As the efficiency of the atmospheric vaporizer decreases, either the exit flow rate or the exit temperature or both must be decreased. Depending on the relative importance of these parameters, one or both of them are decreased until the steady state condition is achieved. Typically, this steady state condition is achieved at about 20 percent of the capacity of the vaporizer without the frost buildup.
When the user's requirements are intermittent, the frost buildup is generally not a problem because whatever frost built up does occur during operation melts off or can be easily removed while the unit is not in operation. Under these conditions, the vaporizer is operating efficiently, and the above-mentioned low efficiency steady state is generally not encountered.
However, when the atmospheric vaporizer is operating in the continuous mode, the frost and ice do not get a chance to melt, and the vaporizer is soon operating inefficiently. For this reason, atmospheric vaporizers are generally not preferred for continuous vaporization of stored cryogenic liquids. Instead, a vaporizer is employed which utilizes a source of heat other than or in addition to ambient heat. This source of heat or energy is normally obtained from steam or electricity. Due to the escalating cost of energy, it is desirable to reduce or eliminate the need for a supplementary heat source of the vaporizer. It would be desirable to vaporize stored cryogenic liquid continuously without encountering the heretofore unavoidable drastic decrease in operating efficiency characteristic of atmospheric vaporizers of the prior art.